Light valves based on liquid crystal display technology and MEMS (micro-electro-mechanical systems) technology have been used in various display applications which include, but are not limited to projectors, projection TVs, camcorders, digital still cameras, internet appliances, cell phones and headsets. To operate, light valves require an illumination system for providing light. The light illuminates the valves, which control how the light is displayed so that viewer can perceive images. Light valves are often microscopic in size, allowing highly detailed images. In most of light valve applications, low cost, compact, light weight illumination systems are desirable. In addition, in such applications, a uniform, bright and stable image is an important characteristic.
FIGS. 1A-1C show cross-sectional views of known projection systems 10, 20 and 220 that include illumination systems utilizing light guide integrators and lens plates in order to provide uniform light to display panels 17, 27 and 227. In FIG. 1A, the input light 13 is focused into the entrance aperture of a straight light guide integrator 14. Light beam 15a exits light guide integrator 14 more uniformly and homogeneously across the exit aperture in terms of spatial light intensity. The exit aperture of light guide integrator 14 is imaged onto the image gate of display panel 17 through a relay lens 16a. A set of lenses may be used to perform the function of relay lens 16a and collimate the light beam 15a. The light beam which passes through a transmissive display panel 17 (FIG. 1A) or gets reflected by a reflective display panel (not shown) is focused by a field lens 16b into the aperture of a projection lens 18, which in turn projects the image displayed on the display panel 17 onto a screen 19. Where projection systems 10, 20 and 220 utilize liquid crystal display (LCD) panels, polarizers and analyzers are usually inserted before and after display panels 17, 27 and 227, in addition to the components shown in FIGS. 1A-C.
The light guide 14 can be straight 14a as shown in FIG. 1D, tapered 14b as shown in FIG. 1E, asymmetrical 14c, as shown in FIG. 1F, or hexagonal 14d, as shown in FIG. 1G. In addition, the light guides 14a, 14b, 14c and 14d can be solid glass rods with polished surfaces, or alternatively, hollow tunnels with reflective surfaces. The light enters the entrance aperture 1, 3 and 6 and emerges from the exit aperture 2, 4 and 7 more uniformly after experiencing multiple reflections, as in case of hollow light tunnels, or multiple total internal reflections, as in case of solid light rods. The light uniformity at the exit aperture increases with an increase in light guide 14a, 14b, 14c and 14d length L.
As shown in FIG. 1D, the entrance 1 and exit 2 apertures of straight light guide 14a have equal W1×W2 cross-sectional areas. Tapered light guides 14b of FIG. 1E deliver uniform and more collimated light in comparison with straight light guides of FIG. 1D. A tapered light guide 14b usually has unequal cross-sectional areas of its entrance A1 and exit A2 apertures. Entrance 3 and exit 4 apertures can have unequal sizes and similar aperture shapes such as square, rectangular or circular as well as unequal sizes and different aperture shapes. Such light guides have been described in U.S. Pat. No. 6,332,688, of Magarill and U.S. Pat. No. 4,765,718, of Henkes, which are hereby incorporated by reference.
Asymmetrical light guides 14c of FIG. 1F are used to compensate for Keystone distortion, illumination overfill regions, and illumination drop-off regions. Asymmetrical light guides have been described in U.S. Pat. No. 6,517,210, of Peterson and U.S. Pat. No. 5,059,013, of Kantilal Jain, both of which are hereby incorporated by reference. The light guide described in the '013 patent places a quartz diffuser 8 with a thickness td at the exit aperture 7 of a hexagonal light guide 14d, as shown in FIG. 1G, in order to convert light emerging from the light guide exit aperture into self-luminous light (i.e., each point over the light beam cross section emits light in a selected range of directions or numerical aperture). Disadvantages of projection system that uses a diffuser at the exit aperture of a light guide are lack of compactness as well as the difficulty of controlling the direction and/or shape of diffused light with most of available diffusers, thus, leading to inefficient light coupling and reduced brightness. In general, known projection systems 10 which utilize light guides 14a, 14b, 14c and 14d to homogenize light suffer from lack of compactness and light losses due to lack of control over the numerical aperture of output light.
In FIGS. 1B-1C, the parabolic 21 and 221 mirror is used to collimate the light emitted by the light source 22 and 222. Lens arrays 23 and 223 divide the substantially collimated input light beam 24a and 224a into sub-beams 24b and 224b. Condenser lenses 25a and 225 focus, in a superimposing manner, the light output from each micro-lens onto the display panel 27 and 227, and in that the width/height ratio of each lens in lens arrays 23a, 23b and 223 corresponds to the width/height ratio of the display panel cross-section in the xy-plane. The light beams which pass through a transmissive display panel 27 and 227 (FIGS. 1B-1C) or get reflected by a reflective display panel (not shown) are focused by field lenses 25b and 228 into the apertures of projection lenses 28 and 229, which in turn project the image displayed on the display panels 27 and 227 onto screens 29 and 230. In projection system 20 of FIG. 1B, two separate lens arrays 23a and 23b are used rather than a single lens array 223 (FIG. 1C). Lens array 223 typically has large number of small-size lenses when compared to lens array 23a and 23b, which requires more precise alignment of lenses on both sides of lens array 223, thus, resulting in difficult manufacturing and more expensive lens arrays 223. Disadvantages of projection systems 20 and 220 include lack of compactness, need for alignment of both plates 23a and 23b and difficulty to control the cone angle distribution (i.e., numerical aperture) of output light 24b and 224b over light valve area, limited light coupling and limited display brightness.
Known projection systems and light integrating technologies suffer from inefficiency in light coupling between components and a lack of compactness. Therefore, there is a need for a compact, light weight, efficient and cost-effective illumination system that provides improved control over spatial distribution of light in terms of intensity and angle over a specific area, such as the active area of a display panel or light valve. Such an illumination system would enable projection systems with smaller light valves arrays, leading to more compact and less expensive projection systems.